Method of watermarking coated paper



Patented Mar. 21, 1944 METHOD OF WATEBMARKING COATED PAPER George 0.Munro, Hamilton. Ohio, asaiznor to The Champion Paper and Fibre Company,Hamilton. Ohio, a corporation of Ohio No Drawing. Application October 2,1940, Serial No. 359,417

6 Claims. (Cl. 117-10) This invention relates to coated paper and to themanufacture thereof and particularly to the marking of such paper foridentification purposes. In the case of uncoated papers this marking isaccomplished by the familiar watermarking process. In making coatedpaper, however, that process of marking cannot ordinarily be usedbecause it has been found that under normal conditions the coatingmaterial applied by the usual brush or roll coating devices fills in,covers, and almost if not completely obliterates any watermarks whichmay be provided irnthe base paper. This is particularly true in thosecases where the paper has been coated on both sides.

Coated paper being used almost exclusively as a base for .the bettergrades of printing, identification marks printed or stamped on, orpressed or embossed into the paper surface could not be used withoutimpairing the value of the paper for its normal use. There has. thus inthe past been no completely satisfactory manner in which coated papercould be marked for identification purposes.

The term coated paper" is used herein as commonly understood in thepaper-and printing industries, to define a fibrous paper base coated onone or both sides with a layer of pigment such form of a series ofrmrailel lines spaced at predetermined intervals. Another of the primaryobjects of the inventionis to provide a method by which suchidentification marks can be applied to coated paper. Another object isto provide a method of applying sufliciently plain marks which shall,however, have only a relatively small effect on the printing surface ofthe paper. A further object of the invention is the adaptation of themethod so that marks of the character described may be applied in theform of a series of parallel lines on the paper. A further object of theinvention is to Provide methcds for this purpose which can be applied asa part of usual paper coating processes and which shall .be simple,effective, and easy to use and control. Other more detailed objects ofthe invention will be apparent from the following description.

as china clay, satin white, calcium carbonate, or

the like, bonded together and to the paper by a water soluble adhesivesuchas starch, casein, glue, or the like, in order to provide asmoother, more uniform surface to receive high grade printing.

There is no limit to the number and variety of markings which may bedesired for identification of the maker, grade, etc., of coated paper.

Certain foreign tariff and import restrictions, however, require thatcoated paper in some classifications be marked with a series of spacedparallel lines. In order that the marked paper may retain its value as abase for printing, it is highly desirable that these lines shall beplainly visible by transmitted light but relatively inconspicuous byreflected light, and that they shall not mar the printing surface of'thepaper to any material extent.

It is one of the primary objects of the invention to provide a type ofcoated paper characterized by identification marks which are plainlyvisible by transmitted light but relatively inconspicuous on the printedand unprinted parts of the surface of the paper when viewed in thenormal manner by reflected light. A further object of the invention isto provide coated paper in which these identification marks are in the Ihave found that, if a steam jet is allowed to impinge on the wet coatingsubstantially immediately after the applying and smoothing operationsbut before any drying operation in the paper coating process, a mark ismade in the coating which remains after the coated sheet is dried andsupercalendered. Examination shows that this mark is due to the force ofthe Jet carrying away the coating material from before it, and either;removing it completely from the web or depositing it on the adjacentparts of the surface. 'I 'his leaves a more or less roughened surface onthe finished paper and thereby seriously impairs its value as a base forhalftone and some other types of fine printing.

I have found, however, that by suitably reducing the force of the Jetand properly choosing and controlling its diameter, the angle at whichit impinges on the paper, and the distance from the formation of the jetto the .surface of the paper, it is possible to prevent any actualphysical displacement of the coating material due to the blowing actionand yet secure a mark which is clearly visible by transmitted light,though scarcely noticeable either on the printed or unprinted portionsof the surface when viewed by reflected light in the normal manner.

Marks made in this manner, though not believed to be due to any actualblowing away of coating material from the mark, nevertheless appear tobe due to a slight local reduction in thick ness of the coating layerbrought about by obscure and little understood causes. Among thesecauses are thought to be local reduction in surface tension due to alocal increase in temperature where the steam contacts the wet coating,

not to rupture the surface film and physically displace the still moreor less fluid coating material. The term "low velocity," as hereinafterused to describe the jets, is accordingly to be understood as definingJets having a velocity insufiicient to rupture the surface film,significantly steam in the jet tends to travel along the surface of thepaper in the same general direction in which the paper is movingrelative to the jet.

That is, the steam advantageously moves with, rather than against, thetravel of the paper past the jet, and the velocity of the steam relativeto the paper is less than it would be if there were no relative movementbetween paper and Jet.

displace the coating therein, and destroy the smoothness or continuityof the surface of the coating. The maximum safe velocity is found tovary with the character of the coating material used. Diiferent types ofcoating composi.-- tions have been used with the process and requiredifferent jet velocities for optimum results. In some cases steampressures as low as one-half pound gage gave what appeared to be theoptimum jet velocity, while with other coatings pressures of two poundswere most desirable, and in exceptional cases pressures as high as fivepounds could be used. Although jet velocities low enough not to rupturethe surface film are ordinarily de sirable, the marking action describedis not dependent on velocity and higher jet velocities may be used incases where the slight surface irregularities resulting therefrom arenot objection-- able.

In order to prevent marring of the coating surface by water droplets orforeign matter, precautions should be taken to have the steam clean anddry. This does not mean, however, that the steam need be free from anyadmixture of air.

In order to make the mark clear and distinct the jet is advantageouslyof small diameter such, for example, as is formed when steam escapesthrough an orifice of one millimeter or less in diameter, orifices ofapproximately 0.75 millimeter diameter having been used with goodeffect.

The greater the distance from the point of formation of the jet to thepoint where it strikes the surface of the wet coating. the broader,fainter andfuzzier the mark obtained. The smaller this distance, on theother hand, the more the mark becomes narrow and sharp or hard and themore probable it is to be too conspicuous on the surface and to visiblyaffect the printing results. I have found in practice that one-half inchis a good distance though it may advantageously be more or lessaccording to the type of paper and coating used and the sharpness anddepth of the mark required.

The direction in which the jet strikes the surface of the wet coating onthe web is also important. If the jet is directed perpendicularlyagainst the surface,.t-he mark on the paper is wide and fuzzy, i. e.indistinct at the edges, due presumably to mushrooming of the jet whereit strikes the surface. A narrower and much, more clean-cut mark issecured if thejet is directed towards the surface at an acute angle. Themagnitude of this angle is not critical. I have found angles from Gil to65 degrees-with the surface to ive excellent results, thoughconsiderable variations from this value are permissible.

In order that a mark, other than a spot, be obtained it is necessarythat there be relative movement between the jet and the surface of thewet coatedpaper. I have found it to be advantageous that the angularityof the jet be so re lated to the direction of this movement that the Inmaking the straight parallel-line markings hereinbefore mentioned it isconvenient to have Jets-one for each line-mounted where the paper leavesthe coating device, e. g. after the last of the smoothing brushes on theusual brush coater, and allow thepaper to travel over the jets on itsway to the dryer. For making curved lines or patterns it may bedesirableto move the jets independently while the paper is eitherstattionary or in motion, or to coordinate simultaneous movements of jetand paper. I

If uniformity of marking is desired, care should be taken to maintainthe uniformity of all conditions of operation. The jets should beprotected against extraneous air currents. The size, angularity and jetvelocity, as well as the distance from the point of formation of the jetto the web, should be maintained at constant values which should he thesame for all jets which are desired to give markings of the samecharacter and intensity.

As an illustration of successful operation of the process the followingexamples are given. In each case the required markings were to be in theform of parallel lines approximately 4 centimeters apart. In one casethe paper was a fibrous coating stock weighing about 43 pounds per ream(500 sheets 25 x 38 inches). This was coated on both sides in aconventional coating machine in which the final smoothing given to thewet coating. was accomplished by laterally reciprocating brushes. Fromthe coater the paper passed in a continuous web to a dryer ofconventional construction. The paper when coated and dried weighed about67 pounds per ream. The coating composition applied conwith enough waterto bring the whole to 40 per cent total solids. Within about 2% inchesof the point where the paper leaves the final smoothing brushes, a steampipe was laid trans versely of and about inch below the web. Holes about0.75 millimeter in diameter were drilled every 4 centimeters in astraight line along the length of this pipe. The pipe was turned sothese holes pointed at an angle of about 62% degrees toward the web andin the direction of its movement. Conventional screens, separators,traps, and drains were installed in a steam line leading to this pipe,in order to insure only clean dry steam being blown against the paper.This line was then connected to a live steam line by means of a needlevalve which facilitated accurate control of the steam pressure andresulting jet velocity. In this case optimum results were secured whenthe steam pressure was regulated to about pound. The coated paper, afterbeing dried and supercalendered, was found to show distinct lines whichof the sheet. The lines were, however, scarcely noticeable either on theprinted or unprinted parts of the surface when viewed by reflected lightin the normal manner. The paper served to satisfy both the customsregulations as to marking and the customer's requirements as to printingquality.

In another case a paper stock weighing about 43 pounds per ream beforecoating and about 69 pounds per ream after coating and drying, wascoated on the same type coater with a coating containing:-

Parts China clay 70 Precipitated calcium carbonate 20 Satin white Caseinwith enough water to bring the whole to about 38 per cent total solids.The size and arrangement of the steam jets was the same as before but inthis case a steam pressure of about 1% pounds was required to give amark of suitable depth and character.

I claim:

1. Method of marking two-side-coated paper with a plurality ofsubstantially parallel lines which shall be plainly visible bytransmitted light but relatively inconspicuous on both printed I andunprinted parts of either surface of the paper when viewed in thenormalmanner by refiected light, in which the paper, as it passes fromthe final operation in the coating process on itsway to the dryingoperation, passes over a plurality of small-area, low-velocity steamjets spaced the same distance apart as the required parallel lines, saidsteam jets being located in close proximity to the path of the paper,all jets being formed at substantially the same distance from the papersurface, and directed against the wet surface of the coating at an acuteangle and in the general direction of the paper travel.

2. Method of marking coated paper with lines which shall be plainlyvisible by transmitted light but shall have a relatively small effect onthe printing surface of the paper, in which the paper, after theapplication and final smoothing of the wet coating material thereon, andbefore it reaches the dryerapasses' by a small-area, lowvelocity steamjet corresponding to each line, said jets being in close proximity tothe paper and directed at an acute angle thereto such that the steamcontacting the paper moves in substantially the same direction as thepaper.

3. Method of watermarking coated paper in which, after the coatingmaterial has been applied to the paper and the smoothing of the wetcoating has been completed but before the coating has been subjected toa drying operation, a smallarea, low-velocity steam jet is directedagainst the surface of the wet coatingat an acute angle with saidsurface, and in which the jet and coated paper are moved relatively toeach other in a path to cause the jet to trace thedesired pattern on thesurface of the coating, the angularity of the get relative to the paperbeing at all times such that the relative velocity of steam and paper isless due to said relative movement between the jet and the paper than itwould be without said relative movement.

4. Method of watermarking coated paper in which, after the coatingmaterial has been applied to the paper and the smoothing of the wetcoating has been completed but before the coat= ing has been subjectedto a drying operation, the paper passes in close proximity to asmall-area, low-velocity steam jet which is directed against the coatedpaper surface at an acute angle and in the general direction of thepaper travel.

5. Method of watermarking coated paper in which, after the coatingmaterial has been applied to the paper and the smoothing of the wetcoating has been completed but before the coating has been subjected toa drying operation, a

small-area, low-velocity steam Jet is directed against the surface ofthe wet coating wherever a mark is to appear on the finished paper.

6. Method of watermarking coated paper in which, after the coatingmaterial has been apcoating.

GEORGE C. MUNRO.

